JP5734285B2 - Magnetic pigment comprising a flaky substrate and a layer of maghemite - Google Patents

Description

  The present invention relates to a magnetic pigment comprising a transparent, flaky, homogeneously composed substrate having two parallel major surfaces, and a coating comprising maghemite, It relates to manufacturing methods and their use.

  The magnetic pigments of the present invention are particularly useful for decorative and security applications that can reasonably combine the magnetic properties of the pigments, particularly the ability to orient in a magnetic field, and the distinctive coloristic properties. It can also be used in other fields where either the magnetic properties or the coloring properties can be of interest.

  Besides acicular magnetic pigments preferably used in magnetic recording media, flaky magnetic pigments have long been known for a variety of applications.

US 3,926,659 discloses mica pigments having a uniform iron-containing layer thereon optionally coated with TiO ₂ or ZrO ₂ or their hydrates, which can be gamma iron oxide (maghemite). In the latter case, the pigment is described as being useful in synthetic resins for the production of household magnets or memory magnets (magnetic recording media). The chromatic properties of these pigments are mainly due to the interference color produced by the TiO ₂ or ZrO ₂ layer, which is slightly shifted by the application of the overlying gamma iron oxide layer. These body colors vary from flesh colored to chocolate brown depending on the thickness of the gamma iron oxide layer.

  This type of pigment does not exhibit attractive coloring properties and sufficiently good magnetic properties to be usable for security applications, where the striking color effects as well as the magnetic effects are significant security benefits. Indispensable for creating features.

In DE10065761A1, flaky magnetic particles are described, which are multi-layered, comprising a core comprising Al ₂ O ₃ or a mixed phase of Al ₂ O ₂ and SiO ₂ , an intermediate layer of amorphous SiO ₂ and iron. Including the containing shell, in particular the latter can be maghemite. These particles are coated with an inorganic or organic coupling agent that can react with nucleic acids or proteins for its isolation in aqueous solution. Since these pigments are made of aluminum powder by suspension in water and addition of water-soluble silicatic compounds, the core is not a homogeneous composition, but rather a mixed oxidation of aluminum and silicon In some cases, the balance being aluminum metal. In addition, the tabular shape of the grains and their smooth surface may not be maintained in the resulting pigment because the core material will at least partially decompose. Furthermore, it is difficult to control the production method, because the reaction of aluminum powder in water is itself extremely exothermic and the subsequent reaction with iron compounds is also dangerous (thermite method).

  The coloring properties of these pigments are not described and their use in security applications is also not described.

  In EP341002B1, a thin film structure with optical variability is disclosed, which includes a substrate and a multilayer thin film metal dielectric interference coating thereon, the metal dielectric interference coating being a magnetic property such as a cobalt nickel alloy and reflective A reflective metal layer of a material having properties is included.

  A similar type of pigment, namely an optically variable magnetic pigment, is also described in EP686675B1. These pigments are coated with a first ferromagnetic layer, a second layer of silica, alumina or a hydrate thereof, a third layer of metal or black metal oxide, and optionally a fourth layer of colorless or colored metal oxide. Made of a layered non-ferromagnetic metal substrate.

  The pigments described in the last two documents have a three-dimensional effect when oriented in different directions by magnetism and show different colors depending on the viewing angle (optical variability), so they are used for security applications can do. They exhibit strong shielding properties due to the incorporated metal layer.

  Most known security applications, especially bank notes, identification cards, etc. require the characteristics described above, but allow for more transparency if necessary, and significant optical There is also a need for more sophisticated smarter effects that exhibit useful color without having variable properties. In particular, a profound golden color is always considered valuable in most cultures and is highly desirable for valuable goods such as banknotes. Benefits such as the combination of such a golden color with a strong magnetic effect, and a clearly visible three-dimensional effect using just a single pigment will be appreciated in the art.

  The object of the present invention is therefore to exhibit magnetic properties sufficiently strong to be easily oriented in a magnetic field to produce a three-dimensional effect, exhibit a golden interference color similar to pure gold, and to some extent transparent It is to provide pigments that exhibit their properties in combination with a comfortable shielding power on the other, an economical method for producing these pigments, and their use.

  The object of the present invention is achieved by a magnetic pigment comprising a transparent, flaky, homogeneously structured substrate having two parallel major faces and a coating comprising maghemite.

In addition, the object of the present invention is a method for producing such a magnetic pigment,
(A) Dispersing, in water, particles composed of a transparent, flaky, homogeneously structured substrate having two parallel major surfaces and optionally coated with at least one dielectric layer ,
(B) adjusting the pH between 2 and 4 and maintaining the pH constant;
(C) adding a water-soluble iron compound while keeping the pH constant;
(D) increasing its pH to a value between 5.5 and 7.5;
(E) optionally adding an aqueous alkaline earth compound solution while maintaining the pH constant;
(F) optionally washing and filtering the resulting product;
(G) drying at a temperature between 100 ° C. and 250 ° C. for a time between 1 and 10 hours, or alternatively (h) at a temperature between 350 ° C. and 450 ° C. for a time between 5 and 30 minutes It is achieved by a method including a step of baking at a.

  Furthermore, the object of the present invention is to pigment, laser mark, and various inks, paints, varnishes, coating compositions, plastics, foils, paper, ceramics, glass, cosmetics and pharmaceutical formulations. It is also achieved by the use of said pigments for coloring pigment preparations with a suitable solvent content.

  A first object of the present invention is to provide a magnetic pigment comprising a transparent, flaky substrate having a homogeneous composition and having two major surfaces parallel to each other and having a coating comprising maghemite thereon. Is achieved by doing

  A flaky substrate in the sense of the present invention is a granular substrate having an upper surface and a lower surface that both constitute the major surface of the granular substrate and are parallel to each other.

  Parallel in the sense of the present invention is not only strictly parallel in the geometric sense, but also has a major surface that is smooth and flat, and an angle of deviation relative to the geometric parallel plane. It also means substantially parallel in the sense that it does not exceed 15 °. The extension in the length and width of the major surface constitutes the maximum dimension (particle size) of the flaky particles. The length difference between the major surfaces constitutes the thickness of the flaky substrate. In general, the thickness of the flaky substrate of the present invention is much smaller than its particle size. Usually, the ratio between the particle size and thickness (aspect ratio) of each single flaky substrate is at least 2, preferably ≧ 20, but even greater than 1000, eg up to 20,000 Good.

  Transparent in the sense of the present invention is a flaky substrate when it substantially transmits visible light, i.e. it conducts at least 90% of incident visible light radiation.

  The base of the magnetic pigment of the present invention is homogeneous in composition, i.e. composed of the same material, either a single compound or a mixture of compounds or mixed oxides, at each position of the substrate. In particular, there are no special regions or gradients of different materials within a single substrate particle.

  Flaky substrate particles of the type described above are usually not available by using common natural substrate particles such as mica, talc or other phyllosilicates. The last material is composed of several layers that are layered on top of each other so that the outer surface of the material is not flat and smooth, but shows steps inside the layer package. Accordingly, planar and extremely smooth surfaces may not be achieved with these materials.

In contrast, synthetically produced substrate materials such as Al ₂ O ₃ , SiO ₂ , glass or different borosilicate substrates can be used to reduce particle thickness, as well as smoothness of the outer surface. By controlling precisely, it can best be produced by ultimately controlling particle thickness variations and particle size deviations. Accordingly, synthetically produced transparent substrates are preferred in the present invention.

Base pigment of the present invention, preferably, based on the weight of the _{Al 2 O 3, Al 2 O} 3 containing TiO ₂ up to 5% by weight based on the weight of the substrate, SiO _2, substrate 20 Made of SiO ₂ , glass or borosilicate containing up to wt% silicon hydroxide. Particularly preferred are Al ₂ O ₃ or Al ₂ O ₃ substrates containing up to 5% by weight of TiO ₂ , both of which are hereinafter referred to as aluminum dioxide flakes. Flaky substrate particles of the SiO ₂ containing silicon hydroxide to SiO ₂ or 20 wt% are called silica flakes hereinafter.

  The substrate used for the pigments of the invention has an average thickness between 100 and 1000 nm, preferably between 150 and 500 nm, most preferably between 200 and 400 nm. The deviation of the thickness of the substrate particles preferably does not exceed 10% and can be controlled by the corresponding method for producing the substrate particles.

  The average diameter of the substrate particles, ie the particle size, corresponding to the largest dimension of the substrate is usually between 5 and 200 μm, in particular between 5 and 150 μm, most preferably between 10 and 100 μm. A narrow particle size distribution is particularly advantageous. The particle size distribution can be controlled by a milling process or a classification process or both.

  Transparent substrates of the type described above also exhibit higher saturation, sharper interference colors, and surprisingly higher magnetism when comparing maghemite coatings to, for example, magnetic pigments on mica substrates. Add to magnetic pigment.

  The coating comprising maghemite (gamma iron oxide), hereinafter referred to as maghemite coating, may be a coating on only one of the major surfaces of the transparent substrate, but is preferably a transparent, flaky substrate The transparent substrate is just encapsulated so that all outer surfaces of the are coated with a maghemite coating. Of course, the maghemite coating need not exhibit the same thickness at each single point on the substrate surface, and of course there may be several small substrate surface areas that are not completely covered by the maghemite coating. That is. This type of limitation is due to technical manufacturing aspects and does not hurt the intent of the present invention. In general, the thickness of the maghemite coating in the magnetic pigment of the present invention is in the range of 20-400 nm, preferably 30-300 nm, most preferably 50-200 nm.

The maghemite coating is preferably a single layer comprising maghemite. In a particularly preferred embodiment of the invention, the maghemite coating is a single layer containing only a single iron compound that is maghemite. Nevertheless, the maghemite coating can also contain various iron compounds in addition to maghemite, such as magnetite (Fe ₃ O ₄ ) or hematite (alpha iron oxide) in minor contents. There are also embodiments of the invention. The last two can be present in the maghemite coating as a single compound or combination, with a total content of up to 25% by weight, based on the weight of the maghemite coating (layer).

  In addition, it is preferable to dope the maghemite coating with at least one alkaline earth metal oxide. Suitable alkaline earth metal oxides for this purpose are Mg, Ca, Sr and Ba oxides or mixtures thereof. Particularly preferred is MgO doping.

  The alkaline earth metal oxides described above, in particular MgO, are present in the maghemite coating, preferably in a content between 0.01 and less than 0.1% by weight, based on the weight of the maghemite coating. They do not form mixed oxides with the iron component, in particular ferrite, because their content is much too low. Instead, they are present in the maghemite coating as alkaline earth metal oxides themselves, for example MgO. Alkaline earth metal oxide doping imparts to the magnetic pigments of the present invention improved dispersibility, better gloss, clearer color and improved stability to heat and weathering. Furthermore, the interference color of the pigment is surprising when the alkaline earth metal oxide is present as a doping in the maghemite coating, as compared to the magnetic pigment of the present invention where the maghemite layer does not contain an alkaline earth metal oxide doping. Not only is it clearer, but also high saturation can be achieved.

  Accordingly, embodiments of the present invention in which the maghemite coating is doped with an alkaline earth metal oxide are preferred. MgO is particularly preferred as the alkaline earth metal oxide.

  Most preferably, the substrate material of the magnetic pigment encapsulates a single substrate encapsulating a substrate that is MgO as the alkaline earth metal oxide doping and maghemite alone as the iron component, preferably no maghemite coating. Fig. 2 is an embodiment of the invention which is an aluminum dioxide flake as defined above having a coating thereon.

  In the last case, the maghemite coating is located directly on the substrate. This embodiment is also preferred for other substrate flakes described above that are different from aluminum dioxide flakes. Nevertheless, to adapt the coloring properties of the magnetic pigments of the present invention and / or to improve the surface properties of the substrate particles and / or to improve the application properties of the pigments in various media. There may be additional coatings (layers) used to improve. To that end, transparent substrate flakes can be coated with at least one dielectric coating prior to coating of the substrate particles with a maghemite coating. Additionally or alternatively, the maghemite coating can be overcoated with one or more dielectric coatings.

As a material for the dielectric coating, a dielectric metal oxide or metal hydrate coating is generally used in the present invention. They can also be colored under certain circumstances, but they can be obtained from colorless metal oxides or hydrates of metal oxides or mixtures thereof, for example, titanium dioxide, zirconium oxide, zinc oxide, tin oxide, It is advantageously constructed from oxides or hydrates of Ti, Zr, Zn, Sn, Ce, Si and Al, such as cer oxide, silicon dioxide and aluminum dioxide or their hydrates.
If the dielectric layer is located directly on the substrate flake under the maghemite coating, the material used for the dielectric layer is not the same as the material used for the substrate (silica flake and dioxide, respectively). It goes without saying that about aluminum flakes.

  The thickness of these dielectric coatings used in addition to the maghemite layer depends on the purpose for which they are used. If the interference color of the new maghemite pigment needs to be adapted to certain requirements, the thickness of the dielectric layer is in most cases between 20 and 300 nm, which under these circumstances is still the interference layer. This is because it acts as an optically active layer. This is achieved in particular when a layer of titanium dioxide, zirconium dioxide or zinc dioxide is used. When they are used to improve the surface quality of the substrate particles or to adapt the magnetic pigment to a specific application media with respect to dispersebility, light resistance, etc., the thickness is It can also be less than 20 nm, in particular between 1 and 15 nm, preferably between 2 and 10 nm. In the latter case, the dielectric layer as it is does not give any interference to the overall pigment system and only improves the properties of the maghemite layer or substrate. Here, in particular, a layer of silicon dioxide, aluminum oxide, cerium oxide and / or tin oxide is used. Of course, an interference-imparting layer as well as a layer for improving adhesion properties can be used together within the scope of one embodiment of the invention.

  In addition to or as an alternative to the inorganic derivative layers described above, thin coatings of organic materials, for example, various organosilanes, organotitanates, organozirconates, improve adhesion capability in various application media Therefore, it can be adhered to the surface of the magnetic pigment of the present invention. Such coatings are known in the art of effect pigments and therefore their deposition is within the ordinary skill of the person skilled in the art.

  Examples of so-called “post-treatments” of effect pigments of either organic or inorganic nature which can be used in the invention described above are described in the following documents: EP0632109, US5,759,255, DE4317019, DE3929423, DE3235017, EP0492223. , EP0342533, EP0268918, EP0141174, EP0764191, WO98 / 13426 or EP0465805; the contents of which are hereby incorporated by reference.

  Preferably, the visible color of the magnetic pigments of the present invention is gold and most preferably shows no or little color flop when viewed from different viewing angles (ie, “optical” (It is not “variably variable”). The visible color of the magnetic pigment of the present invention is the result of combining the interference color of the interference layer (s) with the absorption color of the maghemite layer (maghemite coating). For this purpose, the thickness of the interference layer (or layers if applicable) needs to be adapted to the thickness of the transparent flaky substrate so that golden interference is achieved. The most important interference layer is a maghemite coating, optionally in combination with the various dielectric coatings described above. In addition, the substrate particles also contribute to the interference of the whole pigment due to the materials used, their thickness and due to their flat, smooth and parallel surfaces. In particular, the influence of the substrate particles is particularly important when the average thickness of the substrate particles is within the preferred range of 150 to 500 nm described above. Suitable for achieving golden interference color when substrate particle thickness and maghemite coating thickness, and optionally other dielectric layer coating thicknesses, are selected within the general ranges described above Selecting the thickness is within the skill of one of ordinary skill in the art. On the other hand, the absorption color of the maghemite layer, which is between yellow and dark brown depending on the layer thickness, also needs to be considered. If the interference color of the pigment and the absorption color of the maghemite layer are very close to each other, most preferably they both exhibit a golden hue, it is a great advantage for the resulting chromatic properties of the magnetic pigments of the present invention. In that case, there may be no significant color travel throughout the pigment, but the gold interference color and the gold absorption color of the magnetic pigments of the present invention are visible in a deep golden hue. This can be mimicked by depositing “golden” pigments of different composition. In addition, the magnetic pigments of the present invention combine a high degree of transparency with a sufficiently high shielding power to exhibit a distinct color effect that is clearly visible even when coated on a white substrate material such as banknote paper. In addition, it also shows very good magnetic properties.

  A further object of the present invention is a method for producing the magnetic pigment as described above, which is reliable, economical and easy to control.

Therefore, the following steps:
(A) Dispersing, in water, particles composed of a transparent, flaky, homogeneously structured substrate having two parallel major faces and optionally coated with at least one dielectric layer ,
(B) adjusting the pH value between 2 and 4 and maintaining the pH value constant;
(C) adding a water-soluble iron compound while keeping the pH value constant;
(D) increasing the pH to a value between 5.5 and 7.5;
(E) optionally adding an aqueous solution of an alkaline earth metal compound while keeping the pH value constant;
(F) optionally washing and filtering the resulting product;
(G) drying at a temperature between 100 ° C. and 250 ° C. for a time between 1 and 10 hours, or alternatively (h) at a temperature between 350 ° C. and 450 ° C. for between 5 and 30 minutes A method is found that includes a step of firing in time.

As the transparent and flaky substrate, a synthetic substrate composed of Al ₂ O ₃ , SiO ₂ , glass or various borosilicates is preferably used. In particular, aluminum dioxide flakes, silica flakes, glass flakes or various borosilicate flakes are used, where aluminum dioxide flakes and silica flakes are defined as described above.

  As already described above, these flakes can be produced with good control of shape, thickness, thickness deviation, surface smoothness, planar surface and particle size distribution. The better these conditions are met, the better the quality and reliability of the resulting pigment with respect to color and magnetism.

For example, the aluminum dioxide flakes described above can be produced by the method described in EP763573A2, which is preferred. These substrate flakes contain a small amount of titanium dioxide to facilitate subsequent coating procedures with a dielectric layer or with maghemite coating. These are commercially available from Merck KGaA, Germany under the trade name Xiralic®. However, as known from JP-A 111239/1982, α-Al ₂ O _{3 in} the form of hexagonal flakes having a particle diameter of more than 10 μm and an aspect ratio (particle diameter / thickness) of 5 to 10 , JP-B-72572 / 1991, aluminum flakes such as α-Al ₂ O ₃ flakes having an average particle diameter of 0.5-3 μm, or described in JP-A 39362/1992 Further, aluminum oxide in the form of hexagonal fine tabular grains having a plane perpendicular to the c-axis growing on a flat plate is also useful as the transparent base particle of the magnetic pigment of the present invention.

  Silica flakes that can be used as a transparent substrate for the production of the magnetic pigments of the invention are described, for example, in WO 93/08237. Here, silica flakes made with silicate precursor materials without the addition of soluble or insoluble colorants are particularly preferred. These preferably include coating the endless belt with a precursor material (such as sodium silicate or potassium silicate), solidifying the resulting film after drying, and the resulting solid film with acid. It is produced by treating, washing, and finally separating the membrane from the belt, thereby producing the desired silica flakes. This type of substrate is available as pure silica flakes or, if having at least one dielectric coating, from Merck KGaA, Germany under the trade name Colorstream®. This type of silica flake is preferably used in the present invention.

  Also available on the market are glass or various borosilicate transparent substrate flakes with the desired properties with respect to flat and smooth surfaces, the relatively small thicknesses described above, and small deviations in thickness and particle size Has been.

  The substrate particles described above may optionally be precoated with at least one dielectric layer. To this end, procedures generally known in the art for pearlescent pigments and effect pigments can be used. In particular, wet chemical coating procedures are preferred, and particularly preferred are wet chemical coating methods that use inorganic starting materials, as these methods are easy to handle and control and result in encapsulated substrate particles themselves. It is.

  In general, the wet coating method of coating substrate particles with a dielectric layer, particularly a layer of dielectric metal oxide or metal oxide hydrate, is carried out as follows. The substrate particles are suspended in water, and one or more hydrolyzable metal salts are suitable for hydrolysis and the metal oxide or metal oxide hydrate is platelet-free without secondary precipitation events. Add at a pH value selected to precipitate directly on the (platelet). The pH value is usually kept constant by simultaneous metered addition of base and / or acid. Subsequently, the pigment can be separated, washed, dried, fired if desired, and the firing temperature for the particular coating present can be optimized. In general, the firing temperature is between 250 and 1000 ° C, preferably between 350 and 900 ° C. If desired, following the deposition of the individual coatings, the pigment can be separated and dried and, if desired, calcined before resuspension for further layer deposition by precipitation. In the method for producing a magnetic pigment of the present invention, when the dielectric layer (s) is attached to the substrate particles immediately before the maghemite layer is attached, and when the dielectric layer (s) is attached to the maghemite coating, It is also possible to completely omit the firing step.

  For completeness, the coating of the dielectric layer can also be carried out in a fluidized bed reactor by gas phase coating, here proposed in EP0045851 and EP0106235, for example to prepare lustre pigments It is possible to appropriately use the technology that has been described. However, the wet coating method described above is clearly preferred.

For example, using the wet chemistry method described above, coating of transparent substrate particles with a silicon dioxide layer can be accomplished by the procedure described below. A potassium or sodium silicate solution is metered into a suspension of the material to be coated and heated to about 50-100 ° C. The pH value is kept constant at about 6-9 by simultaneous addition of dilute mineral acids such as HCl, HNO ₃ or H ₂ SO ₄ . As soon as the desired layer thickness of SiO ₂ is reached, the addition of the silicate solution is stopped. The batch is subsequently stirred for about 0.5 hours.

Preferred for the deposition of the titanium dioxide layer is the technique described in US 3,553,001. An aqueous titanium salt solution is slowly added to the suspension of the material to be coated and heated to about 50-100 ° C., and a substantially constant pH value of about 0.5-5 is applied to a base such as aqueous ammonia or alkaline. Maintain by simultaneous metered addition of aqueous metal hydroxide. As soon as the desired thickness of the TiO ₂ precipitate is reached, the addition of both the titanium salt solution and the base is stopped.

This technique, also called a titration method, is noted by the fact that it avoids excessive amounts of titanium salt. This is achieved by supplying the hydrolysis with only the amount per unit time that is necessary for uniform coating with hydrated TiO ₂ and can be received per unit time due to the available surface area of the coated particles. . Accordingly, hydrated titanium dioxide particles that do not precipitate on the surface to be coated are not produced.

  Methods for wet chemical coating of flaky substrate particles for the production of pearlescent pigments are described, for example, in the following documents. DE1467468, DE1959988, DE2009566, DE2214545, DE2215191, DE2244298, DE2313331, DE2522572, DE3137808, DE3137809, DE3151343, DE3151354, DE3151355, DE3211602 and DE3211517.

  Whether coated pre-coated or not, the following procedure is preferably applied in coating the transparent substrate particles described above with a maghemite layer.

  Suspend the substrate particles in water. Preferably, the suspension is heated to a temperature between 75 ° C and 85 ° C. The pH value of the resulting suspension is adjusted to a value between 2 and 4 by addition of acid to the aqueous suspension and kept constant. The soluble iron compound is then slowly metered into the suspension while still maintaining its pH value constant. After completing the addition of the soluble iron compound, the pH is increased to a value between 5.5 and 7.5, preferably between 6.5 and 7.5 (alkaline earth metal oxide is added to the maghemite layer). To be incorporated into). For this purpose, an aqueous solution of an alkaline earth metal compound is slowly metered into the suspension while keeping its pH value constant, and the suspension is preferably kept under stirring for a further 0.5 hour. Then optionally filtered and washed. Advantageously, the addition of the alkaline earth metal compound is combined with a further addition of at least one water-soluble iron compound, in particular an iron (II) compound. Preferably, the iron (II) compound and the iron (III) compound are added simultaneously with the alkaline earth metal compound. The resulting pigment is separated and either at a medium temperature between 100 ° C. and 250 ° C. for a time between 1 hour and 10 hours or at a temperature between 350 ° C. and 450 ° C. for a short time between 5 and 30 minutes. Dry with mist. Optionally, the resulting pigment can be classified to limit the particle size distribution.

  The method described above is preferably carried out under an inert gas atmosphere, for example using nitrogen, argon or the like.

Furthermore, as already described above, in the case where it is used, the alkaline earth metal compound is coated with a group which is coated during the simultaneous metering of further water-soluble iron compounds into the suspension. It is advantageous to adhere to the material. In the last case, the water-soluble iron compound added in the first step is preferably an iron (III) compound, for example Fe (NO ₃ ) ₃ , while used in addition to the soluble alkaline earth metal compound. The resulting water-soluble iron compound contains at least an iron (II) compound, such as FeSO ₄ ^* 7H ₂ O, and may optionally contain an iron (III) compound such as Fe (NO ₃ ) ₃ .

In general, the following water-soluble iron compounds can be used. FeSO ₄ , FeCl ₂ , Fe (NH ₄ ) ₂ (SO ₄ ) ₂ , Fe (NO ₃ ) ₂ , Fe ₂ (SO ₄ ) ₃ , FeCl ₃ , FeNH ₄ (SO ₄ ) ₂ or Fe (NO ₃ ) ₃ FeSO ₄ and Fe (NO ₃ ) ₃ are particularly preferred.

As the alkaline earth metal compound, a water-soluble compound of Mg, Ca, Sr, Ba or a mixture thereof can be used. The kind of alkaline earth metal salt is not particularly limited. Preferably, sulfates, nitrates, hydroxides or halides can be used as long as they are water soluble. Preferred alkaline earth metal compounds include MgSO ₄ , Mg (NO ₃ ) ₃ , MgCl ₂ , MgBr ₂ , MgI ₂ , Mg (OH) ₂ , CaCl ₂ , CaBr ₂ , Ca (OH) ₂ and CaI _2. It is.

Since MgO is the most preferred alkaline earth metal oxide for doping the maghemite layer, the magnesium compounds described above are most preferred, especially MgSO ₄ .

  The content of alkaline earth metal oxide in the maghemite coating is very small, usually not more than 1% by weight, preferably between 0.01 and less than 0.1% by weight, based on the weight of the maghemite layer.

  Such a small content of alkaline earth metal oxide does not result in the formation of mixed oxides in the maghemite layer, but rather a maghemite layer doped with alkaline earth metal oxides, in particular MgO, is provided by this method. .

  As already described above, doping not only imparts better dispersibility, gloss, clearer interference colors or improved stability to heat and weathering to the magnetic pigments of the present invention, The saturation of the pigment obtained was also improved and this was rather unpredictable.

  When the substrate material and the maghemite coating are adapted to each other to exhibit a gold interference color and a yellow-gold absorption color that is very similar to it as described above, there is no or little color flop, and the respective viewing angles. Can achieve highly attractive colored pigments that exhibit pure visible gold color, high saturation, and sufficiently high hiding power to be clearly visible when coated on a white surface . In addition, these very attractive color properties are accompanied by very good magnetic behavior that allows the orientation of the magnetic pigments of the present invention in a magnetic field to achieve a three-dimensional design.

  Furthermore, at least the coloring properties can be further improved by doping the maghemite coating with an alkaline earth metal oxide.

  The magnetic pigments of the present invention having the properties described above are useful for use in valuable sophisticated objects.

  Thus, for coloring inks, paints, varnishes, coating compositions, plastics, foils, paper, ceramics, glass, cosmetics and pharmaceutical formulations, for laser marking, and for pigment preparations of various solvent contents It is an object of the present invention to use the magnetic pigment of the present invention.

  Particularly for security applications, it is preferred to use the magnetic pigments of the present invention in printing inks because the most common method of attaching pigment-containing media is in printing inks. These include printing operations including gravure printing inks such as screen printing inks, intaglio printing inks, offset printing inks, flexographic printing inks, and inkjet printing inks, to name a few, depending on the actual particle size of the magnetic pigment. May include all types of printing inks commonly used.

  A further object of the invention is also a product containing the magnetic pigment of the invention. In general, the pigments of the invention can be applied to any product that can take advantage of one of the properties of the pigments of the invention, i.e., coloring or magnetic properties or both.

  In particular, the pigments of the invention can be used very advantageously in decorative and security products, since these outstanding properties are highly desirable in both areas.

  Security products in the sense of the present invention include, for example, banknotes, checks, passports, identity verification documents, smart cards, driver's licenses, stock certificates, bonds, check guarantee cards, tax banderols, stamps, tickets, Credit card, debit card, telephone card, lottery ticket, gift certificate, packaging material, decoration material, brand product or any other product that requires confidentiality.

It goes without saying that the magnetic pigments according to the invention can be used in admixture with organic as well as inorganic colorants and pigments and in particular with any kind of effect pigments. Organic pigments and colorants are, for example, monoazo pigments, diazo pigments, polycyclic pigments, cationic, anionic or nonionic colorants. Inorganic colorants and pigments are, for example, white pigments, colored pigments, black pigments or effect pigments. Examples of suitable effect pigments are metal effect pigments, pearlescent pigments or interference pigments, which are generally single or multi-coated platelets such as mica, glass, Al ₂ O ₃ , Fe ₂ O ₃ , SiO ₂ Based on. Examples of the structure and specific properties of these pigments are disclosed inter alia in RD471001 or RD472005, the disclosures of which are hereby incorporated by reference.

  In addition, further colorants that can be used in admixture with the magnetic pigments of the present invention are any kind of luminescent colorants and / or pigments, as well as hologram pigments or LCPs (pigments based on liquid crystal polymers). . The magnetic pigments of the present invention can be used in any desired mixing ratio with commonly used and commercially available pigments and fillers. Limitations in the use of the present pigments with other pigments and colorants are provided only if the mixture greatly interferes with or limits the magnetic or coloring properties of the pigments of the present invention.

  The invention is described in more detail in the following examples, but should not be restricted thereto.

Example 1:
100 g of aluminum dioxide flakes (Al ₂ O ₃ with a trace content of TiO ₂ , an average thickness of 220 nm and an average particle diameter of 18 μm) are suspended in deionized water. The suspension is heated to 80 ° C. with stirring. Slowly add nitrogen gas into the reaction vessel. The pH value is adjusted to 3.0 and kept constant by metering acidic compound (HCl, approx. 15% by weight) into the suspension. An Fe (NO ₃ ) ₃ solution (5.65 g Fe (NO ₃ ) ₃ ^* 9H ₂ O in 100 ml deionized water) is added to the suspension while still keeping the pH value constant. The pH value is raised to about 7.0 by adding a basic composition (NaOH, about 30% by weight) to the suspension. An aqueous solution of Mg component and at least Fe component (243 g FeSO ₄ ^* 7H ₂ O in 1000 ml deionized water, 108 g MgSO ₄ ^* 7H ₂ O and 16 g Fe (NO ₃ ) ₃ while maintaining its pH value constant. ^* 9H 2 _O) was introduced slowly metered into the suspension, further held for 30 minutes while then stirring. The resulting pigment is separated by filtration, washed with deionized water and dried in an oven at 200 ° C. for about 4 hours.

  The obtained pigment exhibits a strong gold color with a high brightness and high magnetism. The structure of the iron oxide layer on the substrate was confirmed to be gamma iron oxide (maghemite) by powder X-ray diffraction and thermal analysis (TG / DTA).

Example 2:
The procedure of Example 1 is repeated except that MgSO ₄ ^* 7H ₂ O is not added to the suspension.

  The resulting pigment exhibits a golden glitter color and very good magnetic properties.

Comparative Example 1:
The procedure of Example 1 is repeated except that mica (average particle diameter 20 μm, average thickness 450 nm) is used as the transparent substrate instead of aluminum dioxide flakes.

  The resulting pigment exhibits a low saturation yellowish color and weak magnetism.

Measurement of coloring characteristics:
0.5 g of each pigment obtained in Examples 1 and 2 and Comparative Example 1 is mixed with 9.5 g of standard NC-acrylate lacquer (available from per catalogue Merck KGaA). The resulting mixture is coated on a normal black / white paper strip with a bar coater and dried.

  The chromatic properties of the samples are measured in the black precoat area of the paper strip using a Minolta CR-300 instrument (product of Konica Minolta Holdings, Inc.). The coloring results are as follows.

  From the results in Table 1, it can be clearly seen that the pigment of Example 1 with MgO doping in the maghemite layer exhibits a very strong golden color with high brightness and high saturation. The coloring results of the pigment of Example 2 are somewhat lower in each case when compared to Example 1, but are still sufficient to show a clear and crisp golden interference color with good saturation. In contrast, the sample of Comparative Example 1 only exhibits a low saturation and a bright yellowish color.

Measurement of magnetic properties:
The magnetic properties of the samples obtained in Examples 1 and 2 and Comparative Example 1 were measured using a oscillating magnetometer (type VSM-5, product of Toei Industry Co., Ltd.) and standard procedure (10 kOe magnetic field at room temperature). It was measured.

  The corresponding results are as follows.

  The results shown in Table 2 show exactly the same magnetic properties of the sample of Example 1 as compared to the sample of Example 2, while the magnetic behavior of the magnetic pigment based on the mica of Comparative Example 1 It is considerably weaker than other magnetic pigments based on alumina.

Claims (18)

A magnetic pigment comprising a transparent, flaky, homogeneously composed substrate having two parallel major surfaces, and a coating comprising maghemite ,
Substrate _is made of a _{Al 2 O 3, 5 Al 2} O 3 containing TiO ₂ up to weight%, _SiO 2, SiO ₂ containing silicon hydroxide up to 20% by weight, glass or borosilicate ,
The maghemite coating is a single layer containing maghemite as the only iron compound and is located directly on the substrate
Magnetic pigment . The magnetic pigment of claim 1, wherein the substrate has an average thickness between 100 nm and 1000 nm. 3. A magnetic pigment according to claim 1 or 2 , further comprising one or more dielectric coatings on the maghemite coating. 4. Magnetic pigment according to any one of claims 1 to 3, wherein the maghemite coating is doped with at least one alkaline earth metal oxide. The magnetic pigment according to claim 4 , wherein the alkaline earth metal oxide is selected from the group of oxides of Mg, Ca, Sr and Ba or mixtures thereof. The magnetic pigment according to claim 4 or 5 , wherein the alkaline earth metal oxide is MgO. The magnetic pigment according to any one of claims 4 to 6, wherein the content of the alkaline earth metal oxide is between 0.01 and less than 0.1% by weight, based on the weight of the maghemite coating. Shows a golden interference color, magnetic pigments as claimed in any one of claims 1 to 7. The following steps:
(A) step of the flaky homogeneously configured constructed particles from the base material is dispersed in water in transparency to have a two parallel major surfaces, however, is the _substrate, Al _{2 O} 3, 5 _{Al 2 O 3, SiO 2,} SiO 2 containing silicon hydroxide of up to 20% by weight containing TiO ₂ up to weight percent, and made of glass or borosilicate,
(B) adjusting the pH value between 2 and 4, and maintaining the pH value constant;
(C) adding a water-soluble iron compound while still maintaining the pH value constant;
(D) increasing the pH to a value between 5.5 and 7.5;
(E) optionally adding an aqueous solution of an alkaline earth metal compound while maintaining the pH value constant;
(F) optionally washing and filtering the resulting product;
(G) drying at a temperature between 100 ° C. and 250 ° C. for a time between 1 and 10 hours, or alternatively (h) at a temperature between 350 ° C. and 450 ° C. for between 5 and 30 minutes The manufacturing method of the pigment as described in any one of Claim 1 to 8 including the process baked with time. The method according to claim 9 , which is carried out under an inert gas atmosphere. The process according to claim 9 or 10 , wherein the alkaline earth metal compound of step (e) is added while simultaneously adding further water-soluble iron compounds. An iron compound is iron (III) compound is added in step (c), containing iron compound added in step (e) is at least iron (II) compound, in any one of claims 9 11 The method described. The method according to any one of claims 9 to 12, wherein the alkaline earth metal compound is a magnesium compound. For coloring inks, paints, varnishes, coating compositions, plastics, foils, paper, ceramics, glass, cosmetics and pharmaceutical formulations, for laser marking and for coloring pigment preparations of various solvent contents, Use of a pigment according to any one of claims 1 to 8 . 15. Use according to claim 14 , wherein the ink is a printing ink. A product comprising the pigment according to any one of claims 1 to 8 . The product of claim 16 which is a security product. Banknotes, checks, passports, identity documents, smart cards, driver's licenses, stock certificates, bonds, check guarantee cards, tax bandarrolls, stamps, tickets, credit cards, debit cards, telephone cards, lottery tickets, gift certificates, packaging 18. A security product according to claim 17 , wherein the security product is a material, decoration, branded product or other product requiring security.